With the increasing packing density of electronic circuits, there is a strong need for the miniaturization of electronic components. Accordingly, multilayer ceramic capacitors are rapidly becoming smaller while having higher capacitances and are used in a wide range of applications requiring varying characteristics.
For example, capacitors for medium- to high-voltage applications with voltage ratings greater than 100 V, which are used in equipment such as engine electric computer modules (ECM), fuel injectors, electronically controlled throttles, inverters, converters, HID headlamp units, battery control units for hybrid engines, and digital still cameras, are required to have high capacitances at high electric field strengths.
However, conventional dielectric ceramic compositions are designed for use with DC voltages at low electric field strengths, e.g., about 1 V/μm. If such dielectric ceramic compositions are used in multilayer ceramic capacitors composed of thinner layers at high electric field strengths, they exhibit a noticeable change in capacitance with applied DC electric field (hereinafter “change in DC bias”). This results in the problem of decreased effective capacitance.
The effective capacitance of multilayer ceramic capacitors decreases with increasing change in DC bias. If a multilayer ceramic capacitor used in an electronic device fails to provide the necessary capacitance specified in the design phase, the electronic device may operate unstably and eventually cease to operate.
The use of such multilayer ceramic capacitors as capacitors for medium- to high-voltage applications also encounters the problems of heat generated from densely packed electronic components and severe use environments such as automotive applications. It has therefore been desirable to achieve high capacitances at elevated temperatures at or above 150° C. under an applied DC voltage (DC bias).
Accordingly, it is desirable to use a dielectric ceramic composition that exhibits less change in DC bias, i.e., has a practically sufficient relative dielectric constant at high applied electric fields and a high relative dielectric constant at elevated temperatures at or above 150° C., for capacitors intended for use at high electric fields. As used herein, the term “high electric field” refers to, for example, an electric field strength of 2 V/μm, and the term “high relative dielectric constant” refers to, for example, 2,000.
To solve these problems, PTL 1 discloses a dielectric ceramic composition containing as a main component a compound containing metal elements including at least strontium, barium, lead, bismuth, and titanium. The atomic ratio of these metal elements is represented by the formula (Sr1-v-w-x-yBavCawPbxBiy)TizO3+a, where v, w, x, y, and z satisfy the following conditions:
0.01≦v≦0.05
0≦w≦0.20
0.05≦x≦0.20
0.01≦y≦0.30
1.00≦z≦1.20
v+w+x+y≦0.50
and a satisfies the excess oxygen content. The dielectric ceramic composition further contains a glass component containing at least one of lithium and boron in an amount of 0.1 to 10.0 parts by weight per 100 parts by weight of the major component. The main crystal phase of the dielectric ceramic composition is a perovskite-type crystal phase.
The dielectric ceramic composition disclosed in PTL 1 exhibits a change in relative dielectric constant of within −10% at an applied DC bias of 2 V/μm, which demonstrates good performance. One problem with this dielectric ceramic composition, however, is that the relative dielectric constant is relatively low, i.e., about 1,500. This dielectric ceramic composition is also not intended for use at elevated temperatures since PTL 1 discloses only the change in relative dielectric constant at temperatures up to 85° C.